Polyarylate resins are known as one class of amorphous engineering plastics having excellent mechanical strength. Recently in the electrical and electronic fields, etc., there is a growing desire for a resin for coating films which is excellent in terms of transparency and wear resistance. In the field of electrophotography, among these, use of polyarylate resins having various structures as binder resins for organic photoreceptors is being investigated.
With respect to application of polyarylate resins to electrophotographic photoreceptors, it has been reported that by using U-Polymer (trade name), which is a commercial polyarylate resin, as the binder resin of a photosensitive layer, the sensitivity is improved as compared with the case of using polycarbonate resins used in many electrophotographic photoreceptors (see patent document 1). However, coating fluids prepared by dissolving the polyarylate resin have low stability and there have been cases where production through coating-fluid application is difficult. For overcoming this problem, it has been reported that by using a polyarylate resin obtained using a divalent phenol ingredient having a specific structure as a binder resin, the stability of coating solutions used for producing electrophotographic photoreceptors is improved and the mechanical strength and wear resistance of the electrophotographic photoreceptors are improved (see patent documents 2 to 5).
Meanwhile electrophotographic photoreceptors are repeatedly used in electrophotographic processes, namely, in cycles each including charge, exposure, development, transfer, cleaning, erase, etc., and deteriorate due to various loads imposed thereon during the cycles. Examples of such loads include a high-voltage load due to the corona charging device which is in common use as a charging device, a chemical load due to the ozone and NOx, which are highly oxidative and generate during discharge, an electrical load due to the carriers yielded by image-wise exposure to light, a high-voltage load during transfer, a mechanical load due to toners, carriers, and paper components, and a photochemical load imposed on the photosensitive layer composition by the erase light or external light. Consequently, for inhibiting the image characteristics from fluctuating when an electrophotographic process is repeated many times, it is necessary to heighten the resistance to those loads. With respect to the resistance of photoreceptors to such loads during repeated use, there are cases where even when there is only a slight difference in photoreceptor material, accumulation of slight damage results in damage which affects the image characteristics. In particular, the recent image forming apparatus are increasingly designed to accommodate increases in image quality, and the image characteristics are hence affected even by slight differences in potential. It is therefore necessary to sufficiently take account not only of differences in initial performance but also of differences in performance during endurance. For example, although no report has been made on any influence of by-products contained in the divalent phenol ingredient in the polyarylate resin on the image characteristics during endurance use, the influences thereof must be sufficiently taken into account because the by-products once polymerized and incorporated into the polymer cannot be removed.
Specifically, with respect to by-products in the divalent phenol ingredient in the polyarylate resin in the case where the polyarylate resin is used in the charge transport layer of a multilayer type electrophotographic photoreceptor, no report has been made on not only how the by-products behave in the charge transport layer but also what functions the by-products perform in the vicinity of the charge generation layer/charge transport layer interface. Especially with respect to image defects whereby image residues remain, such as image memory and ghost images, it has been pointed out that charges may be trapped in the vicinity of the charge generation layer/charge transport layer interface to exert influences on the next step (patent documents 6 and 7). The influences thereof are hence a matter of concern.
Meanwhile, from the standpoint of image stabilization, there recently is a desire for a reduction in the dependence of image formation on environmental fluctuations, in particular, humidity. It is becoming especially important that even under such conditions that humidity fluctuates considerably, a constant image density should be maintained over time and the density be kept even throughout the whole image. For example, there is a problem in that during the period when the image forming process is at a stop, the moisture contained, for example, in the toner, and the like of the developing device affects the sensitivity of that portion of the photoreceptor which faces the developing device, thereby causing band unevenness in the images. Among the titanyl phthalocyanines that have hitherto been used extensively as charge generation substances, titanyl phthalocyanine having the Y-type crystal form, which shows highest sensitivity, is known to have a large humidity dependence of electrical properties. Gallium phthalocyanines having a smaller humidity dependence have come to be used as a substitute therefor. In particular, hydroxygallium phthalocyanines having specific crystal forms and showing high sensitivity have been proposed (patent documents 8 and 9). However, there is no report on influences exerted on the electrical properties and image characteristics of an electrophotographic photoreceptor in the case where use is made of by-products contained in the divalent phenol ingredient in a polyarylate resin such as that described above and of the hydroxygallium phthalocyanines.